1. Field of the Invention
This invention relates to curtain coating webs or articles with liquid.
2. Description Relative to the Prior Art
In the art of curtain coating, such as is conducted in the photographic industry for coating layers of materials onto a support web for forming photographic film and paper, it is known that air flow adjacent the curtain can disturb the curtain and cause defects in the coating on the web. It is also known that it is impossible to render a coating room free of air currents because it is necessary to change the atmosphere in the room, it is necessary for people to enter and leave the room through doors, and there are temperature differentials which cause air currents. Likewise, it has been found impossible to render a coating room completely free of dust and such dust gets caught up in the air currents and carried onto the coating liquids.
Attempts have been made to reduce the disturbing effects on the coating of such ambient air currents.
U.S. Pat. No. 4,287,240, issued Sep. 1, 1981 to Thomas R. O'Connor, describes a coating apparatus provided with a protective shield. The coating apparatus therein described includes a hopper for forming a layer of liquid and for forming the layer into a curtain falling under gravity. A web to be coated is trained about a support roller which is disposed with its axis of rotation parallel to the plane of the curtain and so that the curtain impinges on the web while the web is on the support roller. The web approaches and leaves the support roller substantially horizontally. Disposed about the hopper and extending down as far as just above the web approaching the support roller, is a foraminous shield. The shield is substantially box shaped, with its sixth, the bottom, side open. The shield is formed from fine-mesh metal screening and is of double walled construction. The shield was intended to diffuse air currents impinging thereon so that their velocity is decreased, with a resulting decrease in their ability to disturb the flow of coating liquid. Indeed, it has been found that the residual air currents are necessary to prevent the build-up of water or solvent vapor inside the shield, the water or solvent vapor having evaporated from the liquid intended to form the coating. However, it has been found that the protective shield described in U.S. Pat. No. 4,287,240 does no more than was intended of it, namely to reduce, but not eliminate, the effects of the ambient air currents. It has been found that it does not eliminate the adverse effects of currents in air around the hopper and curtain including disturbance of the curtain. Furthermore, the air contacting the liquids in the curtain is the air of the coating room which contains dust particles even though the most stringent efforts may be made to achieve clean air.
The specification of PCT International Patent Application Number PCT/US89/03082 which was published under International Publication Number WO 90/01178, with Kenneth Ruschak named as inventor, describes another form of shield for protecting the flow of liquid on a hopper slide surface. The shield in the Ruschak application is imperforate and is disposed in very close overlying relationship to the liquid flowing down the slide surface of the hopper. The Ruschak shield is intended not only to prevent currents in the ambient air impinging on the liquid on the slide surface, which impact would cause disturbances of the liquid flow which would result in imperfections in the coating on the web, but also to prevent convection currents immediately adjacent the liquid caused by a difference in the temperatures of the liquid and the surrounding air. Even such convection currents have been found to cause disturbances in the liquid flow on the slide surface which appear as imperfections in the coating on the web. The Ruschak shield does not overlay the major portion of even one side of the curtain and does nothing to protect the other side of the curtain. It is intended solely to prevent disturbance of the liquid on the slide surface.
Problems, in the form of defects in the coating, derived from air currents impinging on the liquid on the slide surface and in the falling curtain, and from dust, remain.